BET proteins are proteins encoded by either of the genes BRD2, BRD3, BRD4, or BRDT. Each of these proteins bears two N-terminal bromodomains. Bromodomains comprise of a conserved ˜110 amino acid segment found in at least 42 diverse proteins that specifically interact with acetylated lysines that occur for example on histone tails (Filippakopoulos and Knapp, FEBS Letters, 586 (2012), 2692-2704). Histones are a constituent part of chromatin and their covalent modifications including lysine acetylation regulate gene transcription. Bromodomains are thus believed to regulate transcription by recruiting proteins to genes that are marked with specific patterns of lysine acetylation.
Several published reports have linked the BET protein family to diseases including cancer, metabolic disease and inflammation. Oncogenic fusions of BRD4 or BRD3 and the Nuclear protein in Testis (NUT) gene caused by chromosomal translocations are underlying an aggressive cancer named NUT midline carcinoma (French et al., J Clin Oncol, 22 (2004), 4135-9; French et al., J Clin Pathol, 63 (2008), 492-6). The BRD3/4 bromodomains are preserved in these fusion proteins, and their inhibition either by knockdown or with the selective BET bromodomain inhibitor JQ1 leads to death and/or differentiation of these cancer cells both in vitro and in animal tumour models (Filippakopoulos et al., Nature, 468 (2010), 1067-73). JQ1 and several other selective BET inhibitors have been shown to bind to BET bromodomains and thereby prevent acetyl-lysine binding, which prevents BET proteins from interacting with chromatin and thereby regulating transcription. BRD4 was also identified from an RNAi screen as a target in acute myeloid leukemia (AML) (Zuber et al., Nature, 478 (2011), 524-8). This finding was validated in vitro and in vivo using the BET inhibitor JQ1 and another selective BET inhibitor named I-BET151 that is chemically unrelated to JQ1 (Dawson et al., Nature, 478 (2011), 529-33). These and other studies showed that BET inhibitors have broad anti-cancer activity in acute leukemias, multiple myeloma and other hematological malignancies. In several cancer models an acute downregulation of the oncogenic transcription factor Myc upon BET inhibition has been observed (Delmore et al., Cell, 146 (2011), 904-17; Mertz et al., Proc Natl Acad Sci USA, 108 (2011), 16669-74). More recent studies suggest that the therapeutic potential of BET inhibitors extends to other cancer indications, for example lung and brain cancer.
Another BET inhibitor named I-BET762 that is closely related to JQ1 in chemical structure and the manner in which it binds to BET bromodomains, was reported to modulate expression of key inflammatory genes and thereby protect against endotoxic shock and bacteria-induced sepsis in mouse models (Nicodeme et al., Nature, 468 (2010), 1119-23). This body of data has been used to support the clinical evaluation of the BET inhibitor RVX-208 in clinical trials in patients suffering from atherosclerosis, coronary artery disease, dyslipidemia, diabetes, and other cardiovascular diseases (McNeill, Curr Opin Investig Drugs, 3 (2010), 357-64 and clinicaltrials.gov), Both RVX-208 and I-BET762 have been shown to upregulate Apolipoprotein A-I, which is critically involved in reducing the tissue levels of cholesterol. Finally, BET proteins have been linked to propagation and transcription regulation of several viruses, and therefore it is believed that BET inhibitors could have anti-viral activity (Weidner-Glunde, Frontiers in Bioscience 15 (2010), 537-549).
In summary, inhibitors of BET bromodomains have therapeutic potential in several human diseases.
